The bipolar transistor, for which John Bardeen, Walter Brattain and William Shockley received the Nobel Prize in Physics, is, as is commonly known, an extremely important semiconductor device. Since its invention, the bipolar transistor has been developed to the point where it is widely used in many diverse applications such as computers for both logic and memory functions, and in virtually all modern communications systems.
One of the parameters of interest in characterizing bipolar transistors is the injection efficiency which is the fraction of the total emitter current that is injected into the base. This parameter is of interest because as it increases the common emitter or common base current gain also increases.
It is well known in the art that high injection efficiencies in bipolar transistors can be achieved by using very low doping concentrations in the base region. While a low base doping concentration does increase the injection efficiency, it unfortunately has deleterious side effects. For example, it increases both the base contact and series resistance and also aggravates the base width modulation effect.
An additional parameter of interest is the emitter junction capacitance. This parameter is of interest because it limits the transistor frequency, i.e., the cutoff frequency decreases as the capacitance increases. The capacitance decreases as the doping concentration decreases but the doping concentration can not be made too low or the emitter injection efficiency will be degraded.